Conventionally, acrylic fiber bundle yarn has been widely used as a precursor for producing a carbon fiber bundle. As a process for producing a carbon fiber bundle, a method of obtaining a carbon fiber bundle by a flame resistance impartation step for a heating treatment of acrylic fiber bindle yarn in an oxidizing atmosphere of 200 to 300° C. to give a flame resistant fiber bundle and subsequently by a carbonization step for a heating treatment in an inert atmosphere of 1000° C. or higher is known.
The carbon fiber bundle obtained as above has various favorable physical properties, and thus it is widely used as a reinforcement fiber of various fiber-reinforced composite material and the like. In addition to the conventional use for an airplane or a sports gear, the carbon fiber bundle is also used for an application in construction, civil engineering, or energy-related industry, and thus the demand is now skyrocketing. In order to further broaden the demand, supplying the carbon fiber bundle at lower cost is desired.
In general, a precursor yarn such as an acrylic fiber bundle for producing a carbon fiber is supplied in bobbin-wound form or laminate form after being folded into a box shape. The precursor yarn in such storage form is supplied to various calcination steps such as a flame resistance impartation step or a carbonization step. In order to improve the workability during the calcination step to lower the production cost of a carbon fiber bundle, it is necessary to connect plural precursor yarns and supply them continuously to a calcination step during the production of a carbon fiber bundle in which the precursor yarn is supplied to a calcination step. For such reasons, a technique of connecting a rear end of a precursor yarn which is in the storage form described above to a tip of the following precursor yarn is known (Patent Documents 1 to 3).
Patent Document 2 suggests a method of using a yarn obtained by performing a flame resistance impartation treatment (thermal oxidization treatment) of a rear end of a precursor yarn, a tip of the following precursor yarn, or both ends, in order to prevent thread breaking, during calcination, that is caused by accumulated heat occurring at a connection part between yarns. Patent Document 1 suggests a method of using a yarn which is preliminarily subjected to a flame resistance impartation step when a rear end of a preceding precursor yarn and a tip of the following precursor yarn are connected. Patent Document 3 suggests a method of using a yarn which is preliminarily subjected to a thermal treatment and using a special knotting method.